Several NMR approaches are being used for the study of serine proteinases and their interactions with inhibitors. These include 1H-NMR of low-field N-H peaks in 1H2O, 1H-NMR of histidine C(2)-H peaks in 2H2O, 31P-NMR of phosphorylated enzymes and zymogens, and 13C studies of specifically enriched natural proteinase inhibitors. The proteins being studied include porcine trypsin, bovine trypsin, porcine chymotrypsin, bovine chymotrypsin, and their zymogens. Evolutionary aspects of the charge relay system are being investigated using the bacterial enzyme, alpha-lytic proteinase and the proteinases from Streptomyces griseus. Questions of particular interest are the pK' values of charge relay components, molecular motions affecting the active site, the mechanisms of zymogen activation, and the mechanisms of interaction between serine proteinases and inhibitors.